1. Field of the Invention
The invention relates to novel thermosetting coating compositions based on .epsilon.-caprolactam-blocked isocyanate prepolymers and cycloaliphatic polyamines and use thereof for producing stoving-resistant gravel-proof primer coatings on metal substrates.
2. Description of the Prior Art
Gravel-proof primer coatings on cars have to meet special mechanical requirements-in order to protect the multi-layer coating structure from damage from gravel and ultimately to prevent the car-body sheet from corrosion.
One known solution is to use coating compositions containing polyurethane ureas. Polyurethane urea binders for producing thick coatings must be produced from relatively low molecular components, i.e., blocked NCO prepolymers and diamines. These binders are known and described in the prior art, e.g., in DE-PS 2 131 299. The .epsilon.-caprolactam-blocked NCO prepolymers described in this reference are obtained by reacting aromatic diisocyanates with polymerization or polycondensation products containing hydroxyl groups and are inert towards amines. The compounds containing hydroxyl groups which are used to produce the NCO prepolymers are preferably the polyether polyols known from polyurethane chemistry. Polyester polyols are not mentioned in this connection, apparently because of the possibility of aminolytic side-reactions between the ester bonds and the free amino groups in the activator during storage and cross-linking of the coating compositions. The advantage of the system in the citation lies in the interaction between hard segments (polyurea groups) and soft segments (polyether components) which provide the viscoplastic coating properties for good protection against gravel.
The known binders, however, have the disadvantage of sensitivity to an increase in the stoving temperature. If they are stoved at exactly 160.degree. C., optimum film properties are obtained. If the stoving temperature is increased to only 180.degree. C., the film is permanently soft. If the stoving temperature is 200.degree. C., the film properties become even worse. The result, particularly in the "overspray" region, where the thick layer of anti-gravel coating becomes thin, is often a loss of adhesiveness in the subsequently-applied filler and additional coating layers.
In practice, particularly in car coatings, the stoving temperature cannot always be maintained narrowly between 160.degree. and 170.degree. C., as required for optimum properties of the known coating compositions. There is therefore a demand for "stoving-resistant," coating compositions, which can withstand stoving temperatures of up to 200.degree. C. without adverse effects on the properties of the coatings.
Accordingly, it is an object of the present invention to provide stoving-resistant coating compositions, particularly coating compositions which may used for producing anti-gravel primer coatings.
This objective may be achieved with the coating compositions of the present invention which are described in detail hereinafter.
It has unexpectedly been found that coating compositions based on .epsilon.-caprolactam-blocked NCO prepolymers prepared from aromatic diisocyanates and polyester polyols in combination with aliphatic polyamines yield coatings which, in contrast to the coatings of DE-PS 2 131 299, give very good results in the anti-gravel test approved by the Verband der Deutschen Automobilindustrie, even under extreme stoving conditions (e.g. 200.degree. C. for 20 minutes). Another surprising finding was that the simultaneous presence of ester groups and free amino groups in the compositions does not result in undesired side reactions during the stoving process or in loss of stability of the coating compositions during storage.